Sacroiliac joint (AP oblique view)

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The sacroiliac joint anteroposterior (AP) oblique view of the sacroiliac joint is one of the projections that make up the sacroiliac series.

Indications

This projection examines both left and right sacroiliac joints for comparison purposes in the evaluation of sacroiliitis and ankylosing spondylitis . It is also used to determine any dislocation or subluxation to the joint.

Patient position

  • patient positioned supine on the imaging table with legs extended
  • elevate the side of interest approximately 25 to 30°
  • the patient’s body should be adjusted to allow the body’s long axis to be parallel to the long axis of the x-ray table

Technical factors

  • AP projection
  • centering point
    • perpendicular to the image receptor, centering 2.5 cm medial to the elevated anterior superior iliac spine
  • collimation
    • laterally to include the entire sacroiliac joint of interest
    • superiorly and inferiorly to include the entire joint
  • orientation  
    • portrait
  • detector size
    • 18 cm x 24 cm
  • exposure
    • 75 kVp
    • 25-30 mAs
  • SID
    • 100 cm
  • grid
    • yes                                                              

Image technical evaluation

  • the sacroiliac joint farthest from the IR should be demonstrated open

Practical points

  • the degree of obliquity required to display the SIJ can vary greatly among patients
  • radiolucent support such as a sponge can be used to help elevate the hip and thorax to maintain position
  • a 20-25° cephalad angle centering 2.5 cm medial and 4 cm distal to the elevated anterior superior iliac spine may better demonstrate the sacroiliac joint free from foreshortening